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D. S. Harwood

Publications and source records attributed to D. S. Harwood.

18 recordsLinked to original sources
USGS1993

U-Pb Zircon Geochronology of the Emigrant Gap Composite Pluton, Northern Sierra Nevada, California: Implications for the Nevadan Orogeny

The undeformed Emigrant Gap composite pluton postdates the Lower to Middle Jurassic Sailor Canyon and Middle Jurassic Tuttle Lake Formations. According to earlier workers, these latter formations contain main and late phase Nevadan-aged (155 +/-3 Ma) spaced, slaty, phyllitic, and crenulation cleavage. Recently discovered fossils indicate that the upper part of the Sailor Canyon Formation can be no older than early Bajocian and no younger than Bathonian. The Tuttle Lake Formation stratigraphically overlies the Sailor Canyon Formation and thus probably includes middle to late Bajocian and/or Bathonian strata. The results of U-Pb work suggest that the Emigrant Gap composite pluton is composed of units that range in age from 168 +/-2 Ma (latest Bathonian to early Callovian) to 163-164 Ma (late Callovian). These new data, when combined with observations summarized above, imply that the Tuttle Lake Formation is older than the undeformed oldest unit of the Emigrant Gap composite pluton (i.e., latest Bathonian or early Callovian), and thus was probably deposited and deformed sometime between middle Bajocian and middle late Bathonian time. Hence, the cleavage contained within the Sailor Canyon and Tuttle Lake Formations could not have formed during the Late Jurassic Nevadan orogeny 155 +/-3 Ma as suggested by earlier workers. Within the foothills belt, just to the west of the Emigrant Gap composite pluton, a pronounced contractional deformation occurred sometime between 200 and 163 Ma (Early to Middle Jurassic). This middle Mesozoic deformation apparently was the result of a collision between an oceanic arc and continental North America. Because of the gross similarity in timing of structures produced during this collision and structures in the wall rocks of the Emigrant Gap composite pluton, we suggest that the latter Middle Jurassic structures are also the result of arc-continent collision, albeit a slightly more continentward expression.

Book chapter↗
USGS1979

System dynamics simulation modeling applied to western coal development environmental impact analysis

This report, which was originally prepared in 1977 as an administrative report, evaluates the Geological Survey's first attempt to use simulation modeling in the environmental impact statement (EIS) process. It discusses the background of the modeling project, evaluates the usefulness of that project to the Northern Powder River Basin regional EIS task force, and recommends procedures for implementing future modeling efforts in regional environmental impact analyses.

Open-File Report↗
USGS1971

Continuous magnetic profiles near ground level as a means of discriminating and correlating rock units

Continuous magnetic profiles were recorded by a truck-mounted magnetometer along road traverses over stratified metamorphic rocks and plutonic igneous rocks of the New England Appalachians. The records show a series of distinctive, highly detailed magnetic anomalies which closely reflect the nature and distribution of near-surface bedrock units. The method provides a rapid and versatile means of discriminating bedrock units, which should be especially useful in regions where bedrock exposures are scarce. Under some conditions, analysis of the anomalies may yield critical data on the shape and magnetization of individual rock units. These data should prove useful for deducing geologic structure and for general studies of the magnetization of rocks.

Connecticut, Maine, Massachusetts, New Hampshire, ↗